Resistance welding processes are widely used in the manufacture of sheet metal assemblies, such as automobile and aircraft bodies. Each resistance weld involves a sequence of electrical energy and mechanical pressure steps. The sequence is provided usually by a microprocessor-controlled controller which governs the timing of both the electrical and mechanical steps, and the amount of power to be delivered to the weld during the electrical steps. For example, the controller may establish a selected number of cycles of alternating current power at a selected current, with a selected electrode force for each step of the welding process. In carrying out the welding process, the operator merely presses a button which sets the controller in operation. The controller then responds to an internal program to carry out all the steps necessary for the welding operation. The program for the controller is stored in an internal memory, and it may vary from a relatively few commands to a large number of commands, depending on the simplicity of complexity of the welding operation. A typical microprocessor-controlled controller for resistance type welding machines is disclosed, for example, in U.S. Pat. No. 4,301,351, Mathews, which is assigned to the present assignee.
The controller disclosed in the Mathews patent is controlled by a microprocessor which has programmable read-only memories and random access memories associated therewith. The controller of the patent provides head and pressure commands to the controlled resistance welding machine; and it includes a digital circuit for compensating for line voltage variations and disturbances which digitizes the RMS input voltage and computes the proper firing time to maintain constant power during line voltage fluctuations and disturbances. The firing times are controlled by silicon controlled rectifiers or ignitrons, as described in the patent.
Over-temperature protection of the switching elements in the controller may be provided in the manner disclosed and claimed in U.S. Pat. No. 4,039,928, which also is assigned to the present assignee.
The welding machine to be controlled of the present invention by the controller includes two electrodes which are squeezed in response to commands from the controller against the opposite sides of the workpiece, and an electric current is then passed through the electrodes and through the workpiece to form the weld. The movement of the electrodes and the current through the electrodes are controlled by the controller of the invention. The controller may be set up by the key pad automatically to control for each welding operation the number of cycles of current to flow during the time interval in which the electrodes are squeezed against the opposite sides of the workpiece; the number of current cycles prior to the initiation of the weld; the number of current cycles as the weld is being formed; the number of current cycles during a hold operation; and the number of current cycles for which an "off" condition is to occur between successive welding operations.
The controller of the present invention may also be set up by the key pad automatically to control the amount of power to be delivered before (PRE-SLOPE), during and after (POST-SLOPE) the weld, for each welding operation. Power is linearly increased during the "pre-slope" interval; is held constant during the weld interval; and is linearly decreased during the "post-slope" interval.